1. Field of the Invention
The present invention relates to a piezoelectric element driving circuit with which a driving duration of a piezoelectric element is controlled. More particularly, the invention relates to such a circuit which can attain low power consumption and stabilized operation.
2. Description of the Related Art
Piezoelectric elements have been employed to construct an actuator of a dot impact printer head. Japanese Laid-Open Patent Publication No. 2-119276 discloses a piezoelectric element driving circuit wherein the piezoelectric element and a capacitor are complementarily charged and discharged in response to switching actions of a switching element such as a transistor. The driving circuit achieves low power consumption and low heat generation by controlling both a timing and a duration at which a voltage is applied to the piezoelectric element.
In a circuit configured so that a piezoelectric element is applied with a voltage through a coil to charge the piezoelectric element, charging and discharging periods of time for the piezoelectric element are determined as a function of a product of an electrostatic capacitance c of the piezoelectric element and an inductance L of the coil, as disclosed in co-pending U.S. Application Ser. No. 07/728,592 filed Jul. 11, 1991. In the circuit arrangement disclosed therein, both the charging and discharging periods of time are less than .pi.LC/2. Given that the ON duration of a switching element is shorter than .pi.LC/2 but longer than .pi.LC/3, the piezoelectric element can be charged up to a target voltage due to energy retained in the coil. However, there is a problem that the voltage across the piezoelectric element does not reach the target voltage before expiration of .pi.LC/2, because the switching element is rendered OFF before the voltage across the piezoelectric element reaches a power source voltage. Such a time lag also occurs at the time of discharge of the piezoelectric element, wherein the switching element is rendered ON before the voltage across the piezoelectric element is zeroed. Longer periods of time are necessary to fully charge and discharge the piezoelectric element than the ON and OFF durations of the switching element that controls the charging and discharging of the piezoelectric element.
If the switching element is ON for more than .pi.LC/2, a current flows in a closed circuit comprised of a coil, transistor and a diode after the piezoelectric element has reached the target voltage. Since those elements have resistive components, heat is generated, causing energy on the coil to waste. Consequently, the energy that could have been recaptured by a power source after the transistor is rendered OFF cannot be entirely recaptured thereby. In view of the foregoing, it is desirable that the switching actions performed by the switching element for the purpose of charging and discharging the piezoelectric element be set to a duration exactly equal to .pi.LC/2.
However, if so set, the following problems arise. When a voltage is applied to the piezoelectric element, an electric field is developed between electrodes of the piezoelectric element, causing dielectrics having polarities to rotate. Due to the rotations of the dielectrics, the piezoelectric element extends or restores. Further, heat is generated when the piezoelectric element is extended or restored.
on the other hand, the dielectric constant of a piezoelectric element differs depending on a temperature, so that the electrostatic capacity of the piezoelectric element changes depending on the temperature. Due to the heat generated when the piezoelectric element is actuated, a period of time during which the switching element is rendered ON must be changed. This problem can be solved if the switching transistor is rendered OFF when it is detected that the voltage across the piezoelectric element reaches the target voltage. To this end, the printing head of a dot matrix printer having, for example, twenty-four pins must have the same number of voltage detection circuits in one-time correspondence with the piezoelectric elements coupled to the twenty-four pins. This causes increase of the cost of the printer.